Clinical and experimental investigations suggest that allergen-specific TH2 memory/effector cells reside in the lung and contribute to the persistence and progression of asthma. Recently, a pathological role for CD4+TH17 cells in the regulation of allergic inflammation has been identified; however, the cell types that can secrete IL-17 within the context of TH2 immune responses remain elusive. We have discovered a novel subset of human TH2 memory/effector cells featuring concurrent TH17 and TH2 cytokine production and GATA3 and ROR?t co-expression at a single cell level; termed IL-17-TH2 cells. Mirroring the findings in humans, IL-17-TH2 cells could be found primarily in inflamed lung, but not other lymphoid organs, and persist as the dominant IL-17-producing T cells at the chronic stage of airway inflammation in a mouse model of allergic lung diseases. Furthermore, we found that the cytokine IL-12 and IL-21 are capable of inducing IL-17 and IL-21, while promoting TH2 cytokine production by TH2 memory /effector cells. The treatment of TH2 memory/effector cells with IL-12 induces a significant up-regulation of IRF4 (interferon regulatory factor 4), a critical factor for the development of TH2 and TH17 immune responses. These findings suggest that substantial plasticity exists within CD4+TH2 memory/effector cells and this can be controlled by local inflammatory cues. Notably, we observed that the frequency of circulating IL-17-TH2 cells within total CD4+TH2 memory/effector cell pool is significantly elevated in blood of patients with atopic asthma. In mouse, we demonstrated that antigen specific IL-17-TH2 cells triggered much prominent influx of inflammatory leukocytes, goblet hyperplasia and mucin production compared with those induced only by classical TH2 or TH17 cells. Collectively, these preliminary studies lead to our central hypothesis that IL-12 and IL-21 induce lung resident CD4+ TH2 memory/effector cells to acquire TH17 inflammatory properties, and that such generated IL-17-TH2 cells contribute to the exacerbation of chronic allergic asthma. In Aim 1, we focus on determining the underlying mechanisms by which IL-12 induce IL-17-TH2 cells. We hypothesize that IL-12 induces up-regulation of IRF4 in TH2 memory/effector cells, leading to IL-17 and IL-21 cytokine production. In Aim 2, we will determine the role of IL-21 in regulating the induction and function of IL-17-TH2 cells. We hypothesize that autocrine IL-21 synergizes with IL-12 to enhance the induction and effector function of IL-17-TH2 cells. In Aim 3, we will define the biological relevance of IL-17-TH2 cells in the immunopathogenesis of chronic allergic asthma. We hypothesize that IL-17-TH2 cells trigger diverse chemokine production, leading to the infiltration of heterogeneous inflammatory leukocytes and pathological changes in lung during chronic allergic asthma. We believe that the proposed studies will broaden our views of the inflammatory properties of IL-17-TH2 cells and the inter-relationship between IL-17-TH2, classical TH2 and TH17 cells during chronic allergic inflammation, and thus provide more specific approaches for the treatments in asthma and atopy.